DO NOT HOOK UP A CAR BATTERY TO YOUR UPS!!!!!!!!!!!
Yes it will work. Yes it will kill you. SLA ( Sealed Lead Acid ) batteries, like in your UPS originally, are safe to use indoors. As the name implies, they are sealed. They won't normally vent while being charged. They will vent if they overheat, but that's a safety feature. Car batteries on the other hand aren't sealed. Even those maintenance free ones aren't sealed. Car batteries ( motorcycles too ) will vent gases when charging. These gases tend to be somewhat explosive. Doing this indoors is dangerous. DON'T DO IT.
David